You can add fields from Salesforce to the template using the Merge Field option,

On the Setup Email Content screen, drag and drop the Text or the Text Section tile to the template, and click it, you see an option to add the fields from Salesforce to the email template on the Merge Field list.

You see the fields of the object that you selected in the Setup Audience screen, i.e., if you have selected Contact, you see Contact-related fields in the Setup Email Content screen.